Calcium transients in neutrophils have been measured with the rapid mix flow cytometer. We have observed that the lag time for the response (2 seconds) is longer than the response time of the instrument (300 msecs). The ability to stimulate the cells uniformly has enabled us to detect cell populations in transition between minimal and maximal calcium levels. To pursue this project we are adding a second PMT to the rapid mix flow cytometer to make possible simultaneous analysis of ligand-receptor interactions and cell response.